The present invention relates to a device for improving the lubrication of ball or roll bearings.
In particular, although not exclusively, it applies to the orientation races of orientable blades in wind turbines having a rotor, the rotational axis of which is permanently oriented substantially along the wind direction. More generally, it applies to any turbine with orientable blades, which is driven by a fluid with its rotational axis parallel to the fluid's flow direction, as well as any bearing used under comparable conditions.
Such wind turbines are generally mounted on top a mast and comprise a stator attached so as to be rotatable about the mast axis and thus, to be oriented according to the wind direction, and a rotor supporting a number of blades. For the rotational velocity of the rotor to be controlled and, particularly, so that it does not exceed a certain limit, the blades are pivotally mounted on the rotor so as to be rotatable about their longitudinal axis.
To that purpose, the stator, as well as the blades, are mounted on sets of concentric ball or roll bearing races, wherein each set comprises a support race considered to be fixed, a moving race rotating within the fixed race, and a ball or roller bearing intermediate the fixed race and the moving race, in order to reduce friction between the moving race and the fixed race.
However, such bearings undergo a number of stresses. Since the wind turbine is exposed to whether stresses, these must be protected within sealed housings, especially as they undergo relatively little rotation. Therefore, it is necessary to use seals, for example of the ring type with lips, and to ensure proper bearing lubrication by grease retained by said seals within said housings.
It has been found that blade orientation races, due to their position on the wind turbine and the forces transmitted to the blades, undergo asymmetrical stresses. As these races are always exposed to the wind in the same direction, which is the rotor axis direction and which is therefore parallel to the radial direction of their plane, it is found that with time, the grease will not be distributed uniformly within the bearing along the periphery of the races, but will tend to follow the wind by accumulating on the side not exposed to wind.
Furthermore, the blade orientation races undergo strong mechanical stresses resulting from the flexural moment transmitted to the bearings by the blades, which are arranged perpendicular to the wind direction. This causes a relative movement, other than their rotation, of both races relative to each other.
The moving races also undergo centrifugal forces caused by the rotation of the rotor and blades, which also influence the distribution of grease within the bearing.
These asymmetrical mechanical stresses influence the distribution of grease within the bearings and generate overpressures causing grease leakage through the seals, which are not designed to undergo such pressures.
This causes non-optimal lubrication of bearings and affects the lifetime of races, so that they must be replaced relatively often.
As a consequence, it is necessary to carry out relatively frequent maintenance operations, in particular for cleaning the parts of the wind turbine that are soiled by grease having escaped from the bearings, and to re-inject grease within the latter through openings formed in the fixed races. These drawbacks strongly affect the operating cost of such wind turbines.
The present invention aims at overcoming these drawbacks. It relates to a device for improving the lubrication of a bearing mounted between two concentric races, which swivel, i.e. rotate, about their common axis relative to each another, and bound an annular gap that accommodates the bearing, is filled with grease and is closed by seals, at least one grease tube being provided between the periphery of one of the two races and said annular gap, and being connected to a respective grease reservoir.
Such a device is described, for example, in GB-A-2 055 154.